Means for progressive dielectric heating



March 25, 1952 N|EL$EN 2,590,562

MEANS FOR PROGRESSIVE DIELECTRIC HEATING Filed July 11, 1947 75 H./-.'Power Supply 2. 76 H. 1? P0 wer' supp 5/ WITNESSES:

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ATTORNEY Patented Mar. 25, 1952 MEANS FOR PROGRESSIVE DIELECTRIC HEATINGRussell A. Nielsen, South signer to Westinghouse Pasadena, Calif., as-Electric Corporation,

East Pittsburgh, Pa., a corporation of Pennsylvania Application July 11,1947, Serial No. 760,475 10 Claims. (01. 21947) My invention relates tomeans for utilizing high-frequency dielectric heating for obtaininghigh-speed production of edge-glued wood veneering. More generally, myinvention is directed to means in which glue bonds two or more pieces ofdielectric material together, and can be speedily and properly cured bymeans of dielectric heating.

In edge-gluing wood veneers or strips, it is necessary to cure or setthe glue used to bond the several pieces of wood together in order toimpart strength to the glue lines or joints. In accordance with myinvention, the glue lines are set by passing them through ahigh-irequency electric field having lines of force that generallyparallel the glue lines. It is desirable, in such practice, to use ashigh a voltage as possible across the glue line in order to secure ahigh rate of production. Initially, the glue is wet and cannot withstanda high voltage gradient. However, as the glue in the joints begins toset or cure, it becomes drier and its dielectric loss factor changes ina manner which lowers the rate at which heat is generated in the jointat a given voltage. On the other hand, as the glue sets, the maximumvoltage gradient that it can withstand without arcing or breakdown,increases.

A broad object of my invention is to provide a dielectric heating systemin which a board made from edge-glued pieces can be produced while thepieces making up the board continuously move through theheating-electrodes of the system.

An object of my invention is to provide a dielectric heating system of atype described for the progressive production of boards of edgegluedpieces of wood, the system being characterized by being able to set theglue line bonding the pieces in a minimum of time and with a minimumlength of heating-electrodes.

A more general object of my invention is to provide a dielectric heatingsystem of a type described for making a veneer-board of a typedescribed, which system comprises, in effect, a plurality of pairs ofheating-electrodes arranged along the path of travel of the board, butconnected to a common or single source of highfrequency power throughvoltage controlling means which permits different and controllablevoltages to be applied to the several pairs of heating-electrodes. V

Objects, features and innovations of my invention, in addition to theforegoing, will be discernible from the following description of its Thedescription is to be taken. the accompanying schematic; necessary to anunderstand-- Figure 2 is a perspective view of a second form.

of apparatus embodying my invention.

In the embodiment shown in Fig. 1, the work to be dielectrically heatedis indicated as com-- prising two fiat pieces of wood veneer 2 and 4,.which are to be edgewise secured together at a joint or glue line 6comprising a suitable glue, such as, for example, a resin, that will setor: cure under heat or under heat and pressure. The glue in the joint iscured as the wood pieces. 2 and 4 are continuously moved by anysuitablemeans in the direction of the arrow 8, between a lowerrelatively large grounded flat metal plate or heating-electrode l0 andtwo much narrower fiat metal plates or heating-electrodes l2 and 14arranged successively in the direction of travel of the glue line 6. Theheating-electrodes l2 and I4 lie in a plane, and are spaced so as to berelatively insulated. They are also insulated from the heating-electrode[0.

A source of high-frequency power is connected in any suitable manneracross the relatively insulated heating-electrodes l0 and it. This isshown schematically by means of conductors l6 and I8 representative ofconnections from a suitable power supply. A further circuit-branch 2:],including a variable capacitor 22, is connected to the electrodes I 2and [4 by means of connecting conductors 24. Hence, the voltage betweenthe heating-electrodes l0 and M is' applied to a circuit including, inseries, the circuit-' branch 20 and the spaced heating-electrodes l0 andI2. The result is that the voltage across the heating-electrodes l0 andI2 will be less than that across the heating-electrodes l0 and M becauseof the voltage drop introduced by the capacitor 22. Preferably, but notnecessarily, the conductor [6 is connected to the heating-electrode H ata point near where the glue line emerges from beneath theheating-electrode l4; whereas the circuit 29 is connected at theopposite ,end of the heating-electrode l4. However, broadly, myinvention is not restricted to such precise connections.

Although the heating-electrode It has been shown as a single metallicplate for convenience, itcan be otherwise formed for electricalcooperation with the heating-electrodes I2 and I4.

higher voltage. in that the upper heating-electrodes arewider .so thatwider work can be heat-treated, as for example, a board 25 having aplurality of par- :allel edgewise joints or glue lines 28.

to provide, in effect, a first pair of heating-electrodes l and I2, anda second pair of heatingelectrodes l0 and M, each pair providing adistinct dielectric field. When the glue line B enters the firstdielectric field between th first pair of heating-electrodes H3 and 12,it is preheated and partially dried and partially cured. After suchtreatment, the glue can be heated faster and can withstand a highervoltage-gradient. Hence, the higher voltage provided between the secondpair of heating-electrodes I0 and i l will set the glue line morerapidly. Consequently, the heating-electrodes are arranged to provide asubstantially straight path in which. the wood pieces 2 and 4 movesubstantially rectilinearly; the heating electrode I2 being in a firstportion of the path and the heating electrode is being in a second ordifferent portion of the path.

Since only one glue line is being treated, the upper heating-electrodesl2 and i4 need not be wide. Their lengths, however, are dependent onsuch considerations as the voltage and frequency of the power supply,the power demand, the kind of glue used, the speed of travel of thewood, and similar factors known to the art. In the embodiment shown, thefinal heating-electrode, such as It, is longer than the firstheatingelectrode, such as l2, in instances where the later treatmentrequires a longer time, even with higher voltage gradients.

As an example of an application of apparatus in accordance with thedescribed embodiment, ,4; inch thick boards can be edge-bonded with aurea formaldehyde adhesive by using frequencies in the neighborhood of 5megacycles, and an insulated heating-electrode of about 3 to 4 feet longfollowing an insulated heating-electrode about 1 foot long, both beingfrom inch to inch wide. The voltage across the first pair ofheating-electrodes, such as iii and i2, may

.be in the order of two-thirds of the voltage across the second pair ofheating-electrodes, such as H] and M. To obtain such a division ofvoltage, the capacitance of the capacitor 22 should be about twice asmuch as that between the heating-electrodes. l0 and I2, bearing in mindthe board between the pairs of heatingelectrodes is a factor in thecapacitance. The area of the heating-electrodes l2 and. it can be madesmall if a small capacitance between each pair of heating-electrodes isdesired. By making the capacitor 22 variable, finer adjustments in thevoltage can be obtained. It is to be observed that the reactance of thebranch circuit from the high-frequency ower supply across theheating-electrodes iii and It is capacitive, and

that the reactance or" the branch circuit 24 is of the same sort,namely, also capacitive.

Fig. 2 shows an arrangement, in accordance with my invention, in which,in effect, three dif- "ferent pairs of heating-electrodes provide threedifierent voltages for dielectrically heating traveling work, thevoltages becoming higher as the work becomes better able to withstandthe Fig. 2 also differs from Fig. 1

With reference to Fig. 2, a source of frequency power is connectedacross a highlower table heating-electrode 38 and an upperheating-electrode 32 at the exit end of the heating system. Forpreliminary. heating of-the work,

two other heating-electrodes 34 and 36 are provided successively towardthe feeding end of the heating equipment. The pair of heating-electrodes30 and 34, and the pair of heating-electrodes 30 and 36 are suppliedwith power through circuits 38 and 56, respectively, connected bywire-connection conductors t2 and M, respectively, to the upperheating-electrode 32. The circuits 3% and 40 include variable capacitors46 and 48, respectively, for controlling the voltage that is absorbed ineach. For example, if it is desired to have the voltage cross the pairof heating-electrodes 3B and 3 5 higher than that across 36 and 30, thecapacitance of the capacitor 4G in the circuit 38, should be higher thanthe capacitance of the capacitor t8 in the circuit 40.

While I have described my invention in forms which are now preferred, itis obvious that the forms are subject to wide modification in accordancewith the teachings and principles of theinvention herein disclosed. Thusby reversing the connections or the direction in which the work travels,it will first be subjected to a higher voltage-gradient and then to alower voltage-gradient. It is also apparent that each pair ofheating-electrodes can provide a distinct electric field having avoltage-gradient which is sharply different from any next adjacentdielectric field.. My invention is in the nature of an improvement overthat more broadly disclosed and claimed in the co-pending application ofTheodore P. Kinn, Serial No. 760,348, now Patent No. 2,546,004, filedconcurrently herewith.

I claim as my invention:

1. Dielectric heating equipment comprising a plurality of pairs ofspaced heating-electrodes between each of which a high-frequencyelectric field can be established, each pair comprising an insulatedheating-electrode, said plurality of pairs of heating-eletcrodes beingso arranged that a dielectric material to be heattreated can passsuccessively therebetween, connection means for connecting a source ofhighfrequency power having a first voltage between a first pair of saidplurality of pairs of spaced heating-electrodes, and circuit means,compris ing a voltage controlling connection, for deriving a secondvoltage from said first pair of heating-electrodes and applying saidsecond voltage to a second pair of said plurality of pairs of spacedheating-electrodes.

2. Dielectric heating equipment providing an elongated narrow path forheating particular portions of a dielectric material successively alongdifierent portions of the path, comprising a first pair of spacedrelatively insulated heating-electrodes, one of said heating-electrodesbeing along one side of a first portion of said path and the other ofsaid heating-electrodes being on the other side of said first portion ofsaid path, a second pair of spaced relatively insulatedheating-electrodes, one of said second pair of heating-electrodes beingon one side one. second portion of said path and the other of saidsecond pair of heating-electrodes being on the other side of said secondportion of said path,

said first portion of said path being spaced from frequency voltageacross said first pair of heating-electrodes, a second circuit meanscomprising said second pair of said heating-electrodes for applying ahigh-frequency voltage to said second pair of heating-electrodes, saidsecond circuit means being connected through a voltage controllingconnection across said first pair of heating-electrodes, said first andsecond circuit means being non-resonant at the frequency of thehigh-frequency power supply.

3. Dielectric heating equipment comprising a plurality of pairs ofspaced heating-electrodes between each of which a high-frequencyelectric field can be established, each pair comprising an insulatedheating-electrode, said plurality of pairs of heating-electrodes beingso arranged that a dielectric material to be heat-treated can passsuccessively therebetween, a conveyor for passing such material betweensaid pairs of heating-electrodes, a supply of high-frequency power, afirst circuit means including a first pair of said heating-electrodes,said first circuit means being connected across said high-frequencypower supply, said first circuit means, as a whole, having asubstantially capacitive reactance with respect to the frequency of saidhigh-frequency power supply, and a second circuit means connecting asecond pair of said plurality of pairs of heating-electrodes to andacross said first pair of heatingelectrodes with said second circuitmeans, as a whole, also having the same sort of reactance as the firstcircuit means with respect to the frequency of said high-frequency powersupply, said second circuit means including a voltage regulatingconnection that extends to and between the insulated heating-electrodesof said first and second pairs of heating-electrodes.

4. Dielectric heating equipment as defined in claim 3, but furthercharacterized by a high voltage point of connection from saidhigh-frequency power supply to said first circuit means being made tothe insulated heating-electrode of said first pair of heatingelectrodes, such point of connection being removed from the point ofconnection of said second circuit means to the latter insulated heatingelectrode.

5. Dielectric heating equipment comprising a plurality of pairs ofspaced heating-electrodes between each of which a high-frequencyelectric field can be established, each pair comprising an insulatedheating-electrode, said plurality of pairs of heating-electrodes beingso arranged that a dielectric material to be heat-treated can passsuccessively therebetween, a conveyor for passing such material betweensaid pairs of heating-electrodes, a supply of high-frequency power, afirst circuit means including a first pair of said heating-electrodes,said first circuit means being connected across said high-frequencypower supply, said first circuit means, as a whole, having capacitivereactance with respect to the frequency of said high-frequency powersupply, and a second circuit means connecting a second pair of saidplurality of pairs of heating-electrodes to and across said first pairof heating-electrodes with said second circuit means, as a whole, alsohaving capacitive reactance with respect to the frequency of saidhigh-frequency power supply, said second circuit means including avoltage controlling connection that extends to and between the insulatedheating-electrodes of said first and second pairs of heating-electrodes.

6. Dielectric heating equipment as defined in claim 5, but furthercharacterized by the high voltage point of connection of saidhigh-frequency 6 power supply to said first circuit means being made tothe insulated heating-electrode of said first pair of heatingelectrodes, such point of connection being removed from the point ofconnection of said second circuit means to the latter insulated heatingelectrode.

7. Dielectric heating equipment as defined in claim 6, but furthercharacterized by said circuit means including a variable capacitor insaid connection.

8. Dielectric heating equipment comprisin a plurality of pairs of spacedheating-electrodes between each of which a high-frequency electric fieldcan be established, each pair comprising an insulated heating-electrode,said plurality of pairs of heating-electrodes being so arranged that adielectric material to be heat-treated can pass successivelytherebetween, a conveyor for passing such material between said pairs ofheating-electrodes, a supply of high-frequency power, a first circuitmeans including a first pair of said heating-electrodes, said firstcircuit means being con nected across said high-frequency power supply,said first circuit means, as a whole, having a predominantly capacitivereactance with respect to the frequency of said high-frequency powersupply, a second circuit means connecting a second pair of saidplurality of pairs of heatingelectrodes to and across said first pair ofheatingelectrodes with said second circuit means, as a whole, alsohaving the same sort of reactance as the first circuit means withrespect to the frequency of said high-frequency power supply, saidsecond circuit means including a voltage determining connection thatextends to and between the insulated heating-electrodes of said firstand second pairs of heating-electrodes, and a third circuit meansconnecting a third pair of said plurality of pairs of heating-electrodesto and across said first pair of heating-electrodes, with said thirdcircuit means including a voltage determining connection extending toand between the insulated heating-electrodes of said first and thirdpairs of heating-electrodes.

9. Dielectric heating equipment as defined in claim 8, but furthercharacterized by said third circuit means, as a whole, having apredominantly capacitive reactance with respect to the frequency of saidhigh-frequency power supply.

10. Dielectric heating equipment as defined in claim 9, but furthercharacterized by said connections of said second and third circuit meansincluding a variable capacitor.

RUSSELL A. NIELSEN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,288,269 Crandell June 30, 19422,321,131 Crandell June 8, 1943 2,333,412 Crandell Nov. 2, 19432,354,714 Strickland Aug. 1, 1944 2,401,991 Walton June 11, 19462,434,573 Mann Jan. 13, 1948 1 2,438,498 Geist Mar. 30, 1948 2,453,185Bilhuber Nov. 9, 1948 2,464,404 Gillespie Mar. 15, 1949 FOREIGN PATENTSNumber Country Date 566,927 Great Britain Jan. 19, 1945 577,208 GreatBritain May 9, 1946

